...and so they have to manage themselves
It's not unreasonable for him to give me a hand is it?
The problem is that you are both framing the donkey work of child raising as being your responsibility, with your husband “helping”, when in actual fact you are both equally responsible for your child. This should be the starting point from which you both work, but it appears that your husband considers baby-related work to be your remit. When you frame his input as “giving you a hand”, you are implying the same and validating that idea in his mind.
I’m going to assume you are a SAHM because that’s how your OP reads to me, apologies if that’s incorrect. I think your solution of giving DH a chunk of sleep from 12-6 is sensible (and I’m guessing could sometimes be 11-6 or 12-7 for example if the baby plays ball? Presumably the baby isn’t always awake either side on the nose?).
Having six hours of undisturbed sleep every night is a pipe dream for most parents with a baby and toddler. He is being massively unreasonable to expect more than that when it comes at the expense of you having less than the guaranteed 3 hours you already get, plus a brief snooze in the morning.
He has to be up for work and I’m sure his boss won’t make allowances for tiredness, so I do think that giving him a chunk of sleep is fair. However you also have a job! An extremely important job that requires your full attention. And with a toddler who may not be napping (or maybe not at the same time as the baby anyway) it’s not like you can necessarily catch up on sleep during the day. You have to be on the ball too.
Sorry OP but your DH sounds quite selfish. He needs to adjust the way he views your set up. If he needs the sleep at night, he should be offering to let you catch up at other times. Does he ever take care of the kids in the evenings or a weekends for a few hours to give you time to rest?